U.S. patent number 5,090,002 [Application Number 07/320,197] was granted by the patent office on 1992-02-18 for positioning systems employing velocity and position control loops with position control loop having an extended range.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to William W. Chow, Alan A. Fennema, Ian E. Henderson, Ronald J. Kadlec.
United States Patent |
5,090,002 |
Chow , et al. |
February 18, 1992 |
Positioning systems employing velocity and position control loops
with position control loop having an extended range
Abstract
A position servo system has a position loop and a velocity loop.
The position loop controls the stop-lock condition and provides for
movement control within a range about the stop-lock position. The
velocity circuit is employed for movements outside of the range of
the position servo circuit. When the velocity servo's loop is being
used, a compare circuit compares the servo drive signal from the
velocity circuit with a signal generated by the position servo loop
which is tracking the velocity servo loop. When the compare circuit
finds that the servo drive signals have equal amplitudes, then the
velocity servo loop is disconnected from an actuator with the
position servo loop then connected to the activation for completing
the movement to a desired or target stop-lock position. The
above-indicated servo system controls a topping or fine actuator
carried on a carriage moved by a coarse actuator. The fine and
coarse actuators move along the same axis and transverse to
movement of any work element, such as a record storage disk. The
coarse actuator is continuously slaved to the positioning of the
topping or fine actuator. A relative position sensor disposed
intermediate the fine and coarse actuators supplies a position
error signal for enabling the coarse actuator to continuously
follow the fine actuator. Feed forward signals are supplied from
the fine actuator to the coarse actuator. A preferred embodiment is
shown using an optical disk environment.
Inventors: |
Chow; William W. (Tucson,
AZ), Fennema; Alan A. (Tucson, AZ), Henderson; Ian E.
(Tucson, AZ), Kadlec; Ronald J. (Tucson, AZ) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23245314 |
Appl.
No.: |
07/320,197 |
Filed: |
March 7, 1989 |
Current U.S.
Class: |
369/44.28;
369/30.15; 369/30.17; 369/44.29; G9B/11.035; G9B/11.042;
G9B/21.015; G9B/7.056; G9B/7.066 |
Current CPC
Class: |
G05B
19/232 (20130101); G05B 19/39 (20130101); G11B
21/085 (20130101); G11B 7/0901 (20130101); G11B
11/10556 (20130101); G11B 11/10571 (20130101); G11B
7/08582 (20130101); G05B 2219/41457 (20130101); G05B
2219/42104 (20130101) |
Current International
Class: |
G05B
19/19 (20060101); G05B 19/23 (20060101); G05B
19/39 (20060101); G11B 21/08 (20060101); G11B
11/00 (20060101); G11B 11/105 (20060101); G11B
7/09 (20060101); G11B 7/085 (20060101); G11B
007/085 () |
Field of
Search: |
;369/32,44.25,44.27,44.28,44.29,44.31,44.32,44.35,33,44.34,43
;360/78.01,78.04,78.05,78.06,78.07,78.08,78.09,73.04,77.08 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Envall, Jr.; Roy N.
Assistant Examiner: Dang; Hung
Attorney, Agent or Firm: Somermeyer; H. F.
Claims
What is claimed is:
1. In a position servo system having an actuator movably mounted on
a support and carrying a transducer to be positioned over a
predetermined travel path with respect to a signal storing member
and being disposed in operative relationship to the signal storing
element, the signal storing member having a plurality of
spaced-apart position-indicating indicia along said travel path and
being sensible by the transducer such that the transducer supplies
a local position indication of the positional relationship of the
transducer to any indicium currently being sensed, position error
means being operatively connected to the transducer for receiving
said indication and for supplying a position error signal
indicative of a position of the transducer from a target
indicium;
the improvement including, in combination:
a position servo circuit operatively connected to said error means
and to the actuator for controlling the actuator to maintain the
transducer in a predetermined relationship to an indicium currently
being sensed by the transducer and upon command to move the
transducer along said predetermined path to a first target indicium
within a predetermined number of indicia from the current indicium,
said predetermined number being greater than two;
a velocity servo circuit operatively connected to the error means
and to the actuator for controlling the actuator to move the
transducer a number of indicia from the current indicium greater
than said predetermined number of indicia along said predetermined
path toward a second target indicium; and
switching means operatively interposed between said servo circuits
and said actuator for enabling the velocity servo circuit to
control the actuator for all movements of the transducer over
indicia spaced from said second target indicium more than said
predetermined number of indicia along the predetermined path and
for switching control of the actuator from the velocity servo
circuit to the position servo circuit when the transducer is not
more than said predetermined number of indicia from said second
target indicium.
2. In the position servo system set forth in claim 1, further
including, in combination:
said position and velocity servo circuits respectively supplying
servo drive signals to the actuator for controlling same; and
said switching means, including signal compare means operatively
coupled to said position and velocity servo circuits for separately
receiving the respective servo drive signals and for comparing the
amplitudes of said servo drive signals, when the amplitudes reach a
predetermined amplitude relationship, then said compare means
actuating the switching means for switching control from the
velocity servo circuit to the position servo circuit as an
indication that the transducer is not more than said predetermined
number of indicia from said predetermined indicium.
3. The position servo system set forth in claim 2, wherein said
switching means further includes proximity means operatively
coupled to said position error means for receiving said position
error signal and being responsive to the position error signal for
indicating that the relative position of the transducer with
respect to said second target indicium is approaching not more than
said predetermined number of indicia from said second target
indicium to actuate said compare means to begin comparing said
servo drive signals; and
said compare means including inhibit means for inhibiting
comparison of the servo drive signals until said proximity means
actuates said compare means.
4. In the position servo system set forth in claim 1, further
including, in combination:
command means coupled to said switching means for supplying a seek
command thereto and to said position and velocity servo circuits
for supplying direction and number of indicia to be traversed in
reaching one of said target indicia; and
mode selection means in the command means responsive to a number of
indicia to be traversed during a seek as being not more than said
predetermined number of actuating the switching means to connect
the position servo circuit to the actuator and disconnect the
velocity servo circuit from the actuator, and being further
responsive to the number of indicia to be traversed being greater
than said predetermined number of indicia to actuate the switching
means to initially connect the velocity servo circuit to the
actuator and enabling the switching means for switching control to
the position servo circuit from the velocity servo circuit when the
working object is not more than said predetermined number of
indicia from said target indicium.
5. In a position servo system as set forth in claim 4, wherein said
predetermined number is greater than two indicia.
6. In a position servo system as set forth in claim 4, further
including, in combination:
said switching means, including quiescing means operatively coupled
to said position servo circuit for quiescing operation of said
position servo circuit during the time the velocity servo circuit
is controlling the actuator; and
proximity means operatively coupled to said position error means
for determining when said transducer is approaching an indicium
which is not more than said predetermined number of indicia from
said target indicium and being responsive to such approach for
actuating the quiescing means to remove the quiescence of said
position servo circuit for facilitating switching control of the
actuator from the velocity servo circuit to the position servo
circuit.
7. In a position servo system as set forth in claim 6, wherein said
quiescing means further includes:
a quiescing portion operatively connected to said velocity servo
circuit and being responsive to the switching means switching
control from the velocity servo circuit to the position servo
circuit for quiescing operation of the velocity servo circuit.
8. In a position servo system as set forth in claim 4, wherein said
position and velocity servo circuits separately supply respective
servo drive signals to the actuator for controlling same; and
compare means in said switching means operatively coupled to said
position and velocity servo circuits for receiving the respective
servo drive signals for comparing the energy value thereof for
responding to the servo drive signals have an equal energy value
for indicating that the transducer is said predetermined number of
indicia from said second target indicium for actuating the
switching means to switch control from the velocity servo circuit
to the position servo circuit.
9. In a position servo system as set forth in claim 8, further
including, in combination:
proximity means in the switching means operatively coupled to said
position error means for receiving the position error signal for
indicating when the transducer is approaching the second target
indicium and is nearing a one of said indicia spaced said
predetermined number of indicia from said second target indicium
for actuating said compare means to begin actively comparing the
energy levels of said servo drive signals.
10. In a position servo system as set forth in claim 8, further
including, in combination:
quiescing circuit means in said switching means operatively coupled
to said position and velocity servo circuits for detecting which of
the servo circuits is currently not controlling the actuator for
quiescing a one of the servo circuits which is not currently
controlling the actuator; and
being responsive to the proximity mans to activate the position
servo circuit to generate its servo drive signal for use by said
compare means even through not controlling the actuator.
11. In a position servo system as set forth in claim 1, further
including, in combination:
said position servo circuit including:
an analog signal summer circuit for supplying a servo drive signal
for controlling the actuator and having first and second
inputs;
the position servo circuit being connected to said transducer for
receiving said local position indication;
a signal differentiating circuit receiving said local position
indication for differentiating same for supplying a differentiated
local position indication;
signal polarity reverse means electrically connecting the signal
differentiating circuit to said summer circuit for transferring the
differentiated local position indication to one input of the summer
circuit;
the reference potential means connected to the second input to said
analog summer circuit for supplying a stop lock reference signal to
said summer circuit for maintaining the transducer in a stop lock
condition over one of said indicia;
track movement control means for supplying a position servo control
signal to said analog summer for replacing the positional reference
signal for actuating the positions servo circuit to move the
transducer within said predetermined number of indicia; and
said command means further including a connection to said position
servo circuit for actuating said track movement control means and
said polarity reverse means for switching control of the actuator
from the velocity servo circuit to the position servo circuit for
synchronously reversing the polarity of the differentiated local
position signal with respect to indicia crossings for maintaining a
predetermined error signal as said servo drive signal from the
position servo circuit to the actuator during position servo
circuit controlling operations.
12. In a servo position system set forth in claim 11,
including:
means in the position servo circuit connected to said transducer
for receiving the local position indication and being responsive to
the local position indication indicating about one-quarter indicia
spacing distance from said target indicium for stopping operation
of said reverse means and said track movement control means for
reestablishing a stop-lock condition over said target indicium.
13. In a position servo system set forth in claim 1, wherein said
switching means includes:
velocity means connected to the position error means and to the
velocity servo circuit for measuring the relative velocity of the
transducer to said signal storing member; and
said velocity means having a velocity threshold for indicating a
desired velocity of the transducer over said signal storing member
as the transducer is said predetermined number of indicia from the
target indicium and being operative to detect that said measured
velocity is at said velocity threshold to indicate that said
transducer is said predetermined number of indicia from said target
indicium for actuating the switching means to switch control of the
actuator from the velocity servo circuit to the position servo
circuit.
14. In a position servo system set forth in claim 13, further
including, in combination:
proximity means in the switching means operatively coupled to said
position error means for estimating that the transducer is not more
than predetermined number of indicia from said target indicium;
and
said proximity means being coupled to the velocity means for
activating the velocity means to begin measuring the velocity of
the transducer with respect to the working element.
15. In a position servo system as set forth in claim 14, wherein
said proximity means include:
a further connection from said proximity means to the position
servo circuit and said proximity means being operative to compare
the operation of the position servo circuit with the operation of
the velocity servo circuit and detecting that the operations are
identical for indicating the velocity between the transducer and
signal storing element is a velocity equal to said velocity
threshold for indicating that said transducer is said predetermined
number of indicia from said target indicium on the signal storing
member.
16. In a position servo system set forth in claim 15, wherein said
position error means includes:
a down counter having a count contents representative of a number
of indicia to be traversed to reach a one of said target indicia
operatively connected to the transducer for receiving said local
position indication being responsive to said local position
indication to down count as the transducer passes over indicia in
traveling toward one of said target indicia and generating a count
indicating the position error of the transducer with respect to the
target indicium.
17. In a position servo system as set forth in claim 1, further
including, in combination:
said position servo circuit including:
control means for controlling operation of the position servo
circuit to move the transducer away from a current indicium by
supplying a ramp signal beginning at a reference potential which
represents a stop lock condition over said current indicium and
increasing in amplitude representative of a distance between said
current indicium and said target indicium and resetting the ramp to
a reference potential upon reaching the target indicium, and
further including means for supplying a ramp signal in response to
the switching means beginning at a maximum amplitude and declining
to said reference potential.
18. In a position servo system as set forth in claim 17, further
including, in combination:
said position servo circuit including:
mode control means operatively connected to said transducer and
said position error means for receiving the local position
indication and the position error signal for indicating that the
transducer is one-fourth a distance between two adjacent ones of
said indicia from said target indicium for instituting a stop lock
condition over the target indicium and terminating said ramp
signals.
19. In a position servo circuit as set forth in claim 1, further
including, in combination:
a data recording apparatus including said servo system and having a
moveable optical transducer as said transducer, an optical record
medium as said signal storing member; and
a plurality of spaced apart record tracks on the optical record
medium with each track being a one of said location indicia.
20. In the invention set forth in claim 19, further including, in
combination:
a frame in the record apparatus;
said optical record medium being an optical data-storing disk
mounted for rotation on the frame; indicium and increasing in
amplitude representative of a distance between said current
indicium and said target indicium and resetting the ramp to a
reference potential upon reaching the target indicium, and further
including means for supplying a ramp signal in response to the
switching means beginning at a maximum amplitude and declining to
said reference potential.
21. In the invention set forth in claim 20, further including, in
combination:
coarse servo circuit connected to said moving means for controlling
movements of the head carriage;
a relative position sensor on the head carriage and operatively
connected to the transducer for indicating relative position of the
transducer with respect to the head carriage;
said coarse servo being connected to the relative position sensor
for receiving the signal therefrom for actuating the moving means
to move the head carriage for servoing the transducer to a
reference position on the head carriage; and
an electrical connection between said actuator and said coarse
servo circuit for adjusting operation of the coarse servo circuit
for feed forwarding signals from said actuator to the coarse servo
circuit to move the head carriage faster when the relative position
sensor indicates a greater than a predetermined relative
displacement between said transducer and a head carriage.
22. In the invention set forth in claim 21, further including, in
combination:
said position and velocity servo circuits supplying servo drive
signals for controlling said actuator; and
compare means in said switching means for receiving said servo
drive signals and comparing the RMS values thereof for actuating
the switching means to switch control of the actuator from the
velocity servo circuit to the position servo circuit when the RMS
values of the servo drive signals are equal.
23. In the invention set forth in claim 22, wherein said position
error means includes:
each of said indicia being a track on the data storing disk and
said target indicium being a target track;
a track counter which down counts to zero when reaching said target
track and indicating when the transducer is said predetermined
number of tracks from said target track;
and said track counter being connected to said compare means to
respond to a track count equal to said predetermined number of
tracks from said target track for activating the compare means to
compare the servo drive signals of said position and velocity servo
circuits.
24. In the invention set forth in claim 23, wherein said local
position indication constitutes a sinusoidal alternation when the
transducer traverses between two adjacent ones of said tracks with
a zero crossing at the midpoint between the tracks; and
said compare means being connected to said transducer for receiving
the local position indication and being responsive to a one of the
zero crossings and to said position error signal to activate the
switching means to switch the mode of operation from the velocity
servo circuit to said position servo circuit.
25. In the invention set forth in claim 24, wherein said compare
means activates the switching means to switch control of the
actuator at a velocity of the transducer with respect to the disk
greater than a radial velocity of any run-out of said disk.
26. In a method of positioning a transducer along a path on a
signal storing member wherein the path has a plurality of sensible
spaced-apart location marks;
including the steps of:
detecting the relative position of said transducer along said
path;
relatively moving the transducer along said path from said detected
relative position toward a target relative position such that the
detected and target relative positions are initially separated
along said path by a first plurality of said marks;
during a first portion of said relative movement controlling the
movement according to a predetermined velocity profile;
sensing for a predetermined range relative location along said path
displaced from said target mark by a second plurality of said
marks, said second plurality being less than said first plurality
but greater than two;
upon sensing said range relative location, switching from said
predetermined velocity profile to a predetermined position error
profile for completing the relative movement of the transducer
along said path to the target mark traversing said second plurality
of marks; and
stop locking the transducer and signal storing member at the target
mark using a predetermined position reference signal.
27. In the method set forth in claim 26, further including the
steps of:
in said sensing step, sensing both said predetermined velocity
error profile and said predetermined position error profile and
when the profiles indicate identical movement of the transducer,
then in said switching step, switching from the predetermined
velocity error profile to the predetermined position error profile
for controlling motion of said transducer with respect to the
signal storing member.
28. In the method set forth in claim 26, wherein the transducer is
carried on a moving carriage with the transducer being relatively
moveable with respect to the moving carriage along said path and
the moving carriage being moveable also along said path;
further including the steps of:
sensing the relative position of the transducer with respect to a
reference position with respect to the moving carriage; and
continuously moving the moving carriage for causing the transducer
to always be close to said reference position on the moving
carriage as possible.
29. In the method set forth in claim 28, further including the
steps of:
moving said transducer along the path over not more than said
second plurality of said marks; and
in such movements of less than said second plurality of marks
controlling the movement of the transducer using only the
predetermined position error profile.
30. A servo positioning system having a coarse actuator carrying a
fine actuator, the fine actuator being movably mounted on the
coarse actuator along a first axis, a frame supporting the coarse
actuator for movement along the first axis, RPS position detection
means operatively connected to the fine and coarse actuators for
indicating the relative position with respect to a reference
position relative of the fine actuator to the coarse actuator;
absolute position detection means operatively connected to the
frame and the fine actuator for indicating a relative position of
the fine actuator to the frame;
the improvement including, in combination:
a first positioning servo loop connected to the fine actuator and
to said absolute position detection means for supplying a servo
drive signal to the fine actuator to control short relative motions
between the fine actuator and the frame;
a velocity positioning servo loop connected to the fine actuator
and to said absolute position detection means for supplying a servo
drive signal to the fine actuator to control long relative motions
between the fine actuator and the frame;
control means connected to said first and velocity positioning
loops to switch control there between respectively for said short
and small relative motions between said fine actuator controlling
the actuator between said frame; and
a second positioning servo loop connected to the coarse actuator
and to said RPS position detection means for controlling motions of
the coarse actuator such that the relative position of the fine
actuator with respect to the coarse actuator during positioning
motions of the fine actuator controlled by either said first and
velocity positioning servo loops is continuously servoing toward
said reference relative position.
31. The servo positioning system set forth in claim 30, further
including, in combination:
feed forward means operatively intercoupling said fine actuator and
said second positioning servo loop for supplying a servo
positioning alteration signal such that said second positioning
servo loop supplies a higher amplitude signal to coarse actuator
for preventing the fine actuator from being displaced from said
reference relative position greater than a predetermined distance
whereby the coarse actuator moves faster during periods of time
that that fine actuator is being strongly actuated by either of
said first or velocity positioning servo loops.
32. In the servo positioning system set forth in claim 31, further
including, in combination:
distance to go means in said control means for supplying a motion
command indicating direction of motion and distance to go from a
current relative position of the fine actuator and said frame as
indicated by said absolute position detection means toward a target
relative position of said fine actuator to said frame;
output means in the control means being connected to said distance
to go means for receiving said motion command and supplying said
motion command only to said first positioning servo loop when said
distance to go is less than a predetermined range of motion from
said target position and quiescing said velocity positioning servo
loop for such motion and being further operative when said motion
command indicates motion greater than said predetermined range to
actuate the velocity positioning servo loop for an initial motion
while quiescing the first positioning servo loop; and
mode change means in the control means operatively coupled to said
absolute detection means for receiving an indication therefrom that
said relative position is actuating the fine actuator and causing
said first position servo loop to complete the motion to the target
relative position within said predetermined range.
33. In the servo positioning system set forth in claim 32, further
including, in combination:
proximity means in said control means operatively coupled to said
absolute position detection means for detecting an approach to said
predetermined range and being responsive thereto for activating
said first positioning servo loop to generate and supply a
positioning servo drive signal in preparation for switching from
said velocity positioning servo loop to the first positioning servo
loop.
34. In a servo positioning system set forth in claim 33, further
including, in combination:
said first positioning servo loop having a stop lock mode for
maintaining relative position of the fine actuator with respect to
said frame and a seek mode operative within said predetermined
range and operating under a first positioning motion profile when
switching from said velocity positioning servo loop to the first
positioning servo loop including a profile beginning at an
amplitude corresponding to the amplitude of the servo drive signal
of the velocity positioning servo loop and declining to a reference
potential usable in said stop lock mode; and
a second position motion profile when seeking from initial
stop-lock position which increases an amplitude to a predetermined
value at a target position and having means for reducing the
profile to said reference potential at said target position.
35. In a servo positioning system as set forth in claim 33, further
including, in combination:
said absolute position detection means supplying a digital number
indicative of distance to go;
said control means being operatively coupled to said absolute
position detection means for indicating a commanded distance to go
and supplying the indication to said absolute position detection
means;
said absolute position detection means, including down counting
means, down counting from said commanded distance to go toward
zero, which represents reaching a target position;
wherein each down count of said absolute position detection means
is representative of an addressable stop-lock position of the fine
actuator with respect to the frame as maintainable by said first
positioning servo loop; and
means in the control means limiting said predetermined range to not
being less than two of said stop lock positions.
36. In a servo positioning system set forth in claim 35, further
including signal compare means in said control means and being
operatively coupled to said first and velocity position servo loops
for receiving the servo drive signals therefrom for comparing
amplitudes thereof and indicating when the amplitudes are
equal;
said compare means being operatively coupled to said absolute
position detection means and responsive to said absolute detection
means indicating a distance to go somewhat greater than said
predetermined range to actuate said first positioning servo loop to
supply a pseudo servo drive signal to the compare means for
comparison with the servo drive signal received from said velocity
positioning servo loop and indicating equality at about an
extremity of said predetermined range; and
said compare means being connected to said control means for
actuating the control means to switch between the velocity
positioning loop and the first positioning servo loop to control
the fine actuator.
37. In a servo positioning system set forth in claim 36, wherein
said fine actuator carries a transducer capable of performing
predetermined signal transducing functions:
a signal storing member movably supported on the frame and being
disposed in working relationship to said transducer for enabling
said signal transducing operation, said signal storing member being
movably supported on the frame in a direction transverse to said
first axis and said transverse movement including unwanted
movements along said first axis;
said signal storing member having position indicating marks
disposed along the first axis, said marks being sensible by said
absolute positioning detection means; and
said fine actuator being moveable along the first axis in a manner
to maintain said transducer in working operative relationship to
said signal storing member irrespective of the amount of movement
along the first axis of the transducer.
38. In a servo positioning system set forth in claim 37, wherein
said signal storing member comprises a record disk rotatably
mounted on said frame for rotation transversely to said first axis
and including a plurality of circular record tracks identifiable by
said absolute position detection means;
each of said tracks constituting one of said position indicating
marks;
said transducer being a signal transducer in operative relationship
to said rotating disk and said absolute position detection means
being in operative relationship to said disk through said
transducer such that the transducer not only senses data signals on
the disk, but also said position indicating marks.
39. In a servo positioning system set forth in claim 38, wherein
said transducer is a set of optical elements focussed on said disk
surface for optically sensing said data signals and said marks;
and
said disk being optically recordable and readable by said
transducer means.
40. In a positioning servo circuit, the improvement, including, in
combination:
position detector means for indicating a relative position between
two members which are relatively moveable with respect to each
other, a plurality of relative position indicating marks on said
relatively moveable members which are sensible by said position
detector means;
an actuator operatively coupled to said relatively moveable members
for relatively moving same;
first and second servo positioning circuit means, each for
supplying a servo drive signal to said actuator for relatively
positioning the two relatively moveable members and being coupled
to the position detector means for receiving said indication of
relative position for controlling the relative positioning of said
relatively moveable members;
output means electrically coupled to both the circuit means for
receiving a servo drive signal therefrom and to said actuator for
supplying one of said servo drive signals thereto;
command means for actuating the first servo positioning circuit
means to supply a first servo drive signal to said output means for
relatively moving the relatively moveable members toward a
predetermined relative position and for actuating the second servo
positioning circuit means to generate a second servo drive signal
as if it were relatively moving the relatively moveable members
toward said predetermined relative position; and
mode switch means coupled to said first and second servo
positioning circuit means, to said position detector means and to
said output means for receiving the servo drive signals from both
said servo positioning circuit means and connected to said command
means for coupling said first servo drive signals to the output
means and being jointly responsive to said servo drive signals and
to said indication of relative position to substitute said second
servo drive signal for the first servo drive signal at said output
means when a given number of said position indicating marks are
disposed between said predetermined relative position and a current
indicated relative position, said given number being greater than
one.
41. In a positioning servo circuit set forth in claim 40, further
including, in combination:
said first servo positioning circuit means having a positioning
control capability of initiating and controlling short relative
motions of said two relatively moveable members and for maintaining
a stop lock relationship between the two relatively moveable
members with respect to any one of said position indicating marks;
and
said second servo positioning circuit means having a capability of
relatively moving said two relatively moveable members over a
greater number of said position indicating marks during a given
single movement, but not having a capability of completing the
movement to a target one of said position indicating marks and
further having the capability of relatively moving the two
relatively moveable members at a faster speed than said first servo
positioning circuit means moves said objects during short relative
motions.
42. In a positioning servo circuit set forth in claim 40, wherein
said second servo positioning circuit means includes means
establishing motion control using a velocity profile.
43. In a positioning servo circuit set forth in claim 40, further
including, in combination:
said position detector means indicating said relative position of
said members by an electrical digital output signal indicative of a
distance from a predetermined target one of said position
indicating marks, having means for down counting said digital
output as the two relatively moveable members approach said target
relative position of said target position indicating mark, and
supplying a proximity signal indicative when said first servo
positioning circuit means can complete any motion or start any
motion which is one of said short relative motions and;
compare means in said mode switch means operatively coupled to said
first and second servo positioning circuit means and to said
position detector means for responding to said proximity signal to
receive the servo drive signals from both of said first and second
servo positioning circuit means for comparing same and for
indicating a time when the amplitudes of said servo drive signals
are equal for switching the coupling from said second servo
positioning circuit means to said first servo positioning circuit
means for completing the movement as a short relative motion.
44. In a positioning servo circuit set forth in claim 43, wherein
said compare means includes amplitude compare means for comparing
the signal amplitudes of said servo drive signals.
45. In a positioning servo system having a signal storing member, a
frame movably mounting a signal storing member along a
predetermined path, a first actuator on the frame and connected to
a fine carriage, the fine carriage being movably mounted on the
frame for carrying a transducer along a given path which extends
transversely to a said predetermined path and being disposed with
respect to the signal storing member to relatively move the
transducer and signal storing member along said given path keeping
the transducer and signal storing member in operative juxtaposition
such that the signal storing member cooperates with the transducer
to perform desired machine operations, error means coupled to the
transducer and the signal storing member for generating and
supplying a position error signal (PES) which indicates a relative
position error between the signal storing member and the transducer
with respect to a desired relative position between the transducer
and signal storing member;
the improvement including, in combination:
an electronic switch means having an output portion being
operatively connected to the actuator and first and second input
portions for receiving servo drive signals;
a first position servo circuit means having a stop lock mode and a
seek mode and being operatively connected to said first input
portion and to said error means for receiving PES and being
responsive to PES for actuating said actuator to relatively move
the transducer in said seek mode within a first range of motion
with respect to said desired relative position which range is
outside of operation of said position servo circuit means in said
stop lock mode and to maintain said relative position of said
transducer and signal storing member in said stop lock mode;
a velocity servo circuit means operatively connected to said second
input portion and to said error means for receiving PES and having
a command input portion for receiving commands to actuate the
actuator to relatively move the transducer and the signal storing
member from said desired relative position toward but not reaching
a target relative position along said given path, said target and
desired relative positions being separated along said given path a
distance greater than said first range;
command means operatively connected to said electronic switch means
nd to said command input portion for actuating the electronic
switch to receive inputs for the actuator through said second input
portion and not through the first input portion and for actuating
the velocity servo circuit means to cause the actuator to
relatively move said transducer and said signal storing member
toward said target relative position; and
restore means operatively connected to both said servo circuit
means and to said electronic switch means for responding to
predetermined operational states of both said servo circuit means
before said transducer and signal storing member have reached an
immediate proximity of said target relative position for actuating
said electronic switch means to again receive input through said
first input portion and not the second input portion at the onset
of reaching the first range and such that the position servo
circuit means completes controlling the actuator during the
relative motion of said transducer and signal storing member to
said target relative position and then institutes stop-lock mode
upon reaching the target relative position.
46. In the positioning servo system of claim 45, further including,
in combination:
a coarse carriage movably mounted on the frame for movement along
said given path, having a reference mark thereon and carrying said
fine carriage along said given path, and carrying said first
actuator;
relative position sensing means on said coarse carriage and
operatively coupled to said fine carriage for indicating the
relative position of a fine carriage with respect to said reference
mark and supplying a position error signal;
a second position servo circuit means operatively coupled to said
relative position sensor for receiving said position error signal
and being responsive to the position error signal for supplying the
coarse servo drive signal;
a coarse actuator on the frame operatively coupled to said coarse
carriage and to said second position servo circuit means for
receiving said coarse servo drive signal and responsive thereto for
causing the actuator to move the coarse carriage along the given
path for reducing the position error signal from the relative
position sensor to zero, irrespective of whether said first
position servo circuit means or said velocity servo circuit means
by actuating the first actuator.
47. In the positioning servo system set forth in claim 46, further
including, in combination:
feed forward means operatively electrically coupling said first
actuator to said second position servo circuit means for supplying
a signal to said second position servo circuit means in proportion
to the signal received by the first actuator from said electronic
switch means output portion whereby the coarse actuator actuates
the coarse carriage for faster motion in proportion to the
displacement of the transducer from said reference mark.
48. In the positioning servo system set forth in claim 47, further
including, in combination:
a plurality of position indicia on said signal storing member in
sensible relationship to the transducer for indicating the relative
position of the transducer to the signal storing member and
sensible by the error means generating said PES; and
mode switch means in said first position servo circuit means
operatively coupled to said error means for responding to PES when
said first position servo circuit means is in said seek mode within
the first range of motion and the relative position of said
transducer and said signal storing member along said given path is
about one-fourth the space between two adjacent ones of said marks
from a target one of said marks from a desired relative position of
said transducer and said signal storing member.
49. In the positioning servo system set forth in claim 48, further
including, in combination:
said signal storing member being an optical record medium movably
mounted along said predetermined path which is transverse to said
given path; and
an optical lens in said transducer in optical communication with
the record medium for optically receiving signals recorded on the
record medium and for optically sensing said marks.
50. In the positioning servo system set forth in claim 49, wherein
said optical record medium is a circular record storage disk
rotatably mounted on the frame for rotation transverse to said
given path, and wherein said predetermined path is radially of said
disk; and
a plurality of concentric record tracks on the record storage disk
each of which include one of said marks for indicating the radial
position of the transducer with respect to the record storage
disk.
51. In the positioning servo system set forth in claim 50, wherein
said first position servo circuit means having first and second
positioning motion profiles, a first of the motion profiles causing
relative motion of the work object with respect to the record
storage disk beginning at an amplitude corresponding to the
amplitude of a servo drive signal supplied by said velocity
positioning circuit means and declining to a reference potential
suitable for use in said stop-lock mode; and
said second position profile beginning at said reference potential
in said stop-lock mode and increasing to a maximal value as
position error increases and then reducing immediately to said
reference potential representative of arriving at said target
position.
52. In a positioning servo system having a first actuator movably
mounted on a support and carrying a transducer with respect to the
support and a signal storing member, said signal storing member and
the support being mounted on a common frame such that the
transducer is moveable by the actuator along a predetermined path
along the signal storing member continuously in an operative
position, a plurality of path position indicating marks on the
signal storing member evenly spaced along the path and means in the
transducer for sensing the marks, error means operatively connected
to the sensing means for generating a position error signal (PES)
indicative of the relative position of the transducer with respect
to the marks disposed along said predetermined path;
the improvement including, in combination:
means for supplying movement commands for relative movements of
said transducer with respect to the signal storing member including
an indication of direction and a number of said marks to be
traversed in the commanded movement from a mark currently being
senses by said sensing means;
command directing means connected to the supplying means for
generating a first long seek command indicating movement of the
transducer to a target position along said path only when said
number exceeds a predetermined number greater than two and a second
short seek command when said number is less than said predetermined
number;
velocity circuit means having a velocity control and being
connected to said actuator for supplying a servo drive signal to
said actuator and being connected to the directing means for
responding to the first long seek command to cause the actuator to
move the transducer in a direction indicated by the long seek
command while using said velocity control;
first positioning circuit means having a positioning seek and
position maintenance control and being connected to said actuator
for supplying a servo drive signal thereto and being connected to
the directing means for responding to the second short seek command
to cause the actuator to move the transducer in a direction
indicated by the second short seek command using said positioning
control and having a stop-lock mode in the positioning control for
commanding the actuator to maintain current position of the
transducer with respect to the signal storing member, stop means
operatively connected to the error means by responding to PES to
institute the stop-lock mode at said target position; and
mode changing means connected to said velocity circuit means and to
said first positioning circuit means for measuring the circuit
operations of said velocity control and said positioning control
and when the measured operations indicate that the transducer is
being moved by the actuator using the velocity control and the
transducer is not more than said predetermined number of marks from
an end of the commanded movement inactivating the velocity control
and activating the positioning control to complete the movement of
the transducer including moving the transducer over at least one
mark intermediate a current position of the transducer and then end
the commanded movement in the stop-lock mode.
53. In the positioning servo system set forth in claim 52, further
including, in combination:
a frame in said system;
said frame movably supporting said support for movement along said
predetermined path in reciprocating motions;
positioning circuit means on the frame;
a coarse actuator mounted on the frame and to the support for
moving the support reciprocally along said predetermined path and
being operatively connected to said second positioning circuit
means for enabling the second positioning circuit means to control
the movement of the support along the predetermined path;
a relative position sensor mounted on the support and operatively
coupled to said transducer for indicating the relative position of
the transducer with respect to the support and being connected to
said second positioning circuit means for indicating a position
error such that the second positioning circuit means continuously
servos for making a relative position error equal to zero such that
the moveable support continuously follows the motion of said
transducer.
54. In the positioning servo system set forth in claim 53, further
including, in combination:
feed forward means operatively intercoupling said first and second
positioning servo circuit such that said first positioning servo
circuit means supplies the servo positioning alteration signal to
said second positioning servo circuit means whereby said second
positioning servo circuit means supplies a higher amplitude
electrical drive signal to said coarse actuator whenever the
transducer is displaced from a reference relative position on the
support greater than a predetermined distance for causing the
coarse actuator to move said support along said predetermined path
faster during periods of time for preventing the transducer from
having a greater than a predetermined relative displacement with
respect to the support.
55. In the positioning servo system set forth in claim 54, further
including, in combination:
proximity means in said load changing means and connected to said
error means for responding to PES to detect an approach by said
transducer to a mark which is displaced from the commanded movement
by said predetermined number of marks for activating the first
positioning servo circuit means to generate and supply a
positioning servo drive signal in preparation for switching from
the velocity positioning circuit means to the first positioning
circuit means and including means for quiescing the operation of
the first positioning circuit means when the velocity circuit means
is operative and before proximity of said transducer reaching said
predetermined number of marks from an end of the commanded movement
is detected.
56. In the servo positioning system set forth in claim 55, further
including, in combination:
said work or element being an optical disk rotatably mounted on the
frame for rotation across said predetermined path;
said transducer including an optical beam steering means for
sweeping a beam along the predetermined path with the position of
the beam on the optical disk being the relative position of the
transducer with respect to said support; and
said optical disk having a plurality of concentric record tracks
facing said transducer and including physical indicating means
constituting said indicating marks.
57. In a positioning servo for moving a transducer along a
predetermined travel path on a signal storing member, a plurality
of spaced-apart marks being disposed on the signal storing member
along said path for indicating a like plurality of positions of
said transducer on the path;
the improvement including, in combination:
a position detector coupled with the transducer and said signal
storing member for detecting and indicating position of the
transducer with respect to a closest one of said marks and
supplying a position error signal (PES) indicating said relative
position;
servo circuit means coupled to the detector for receiving PES and
having a summer circuit for combining the received PES with a
command signal for causing the servo circuit means to move the
transducer to a target position in accordance with the command
signal by combining the command signal with PES in the summer
circuit; and
first, second and third command means coupled to the summer circuit
for respectively supplying a command signal as a predetermined
reference potential for causing the servo circuit means to position
the transducer at a current one of the marks, to move the
transducer from said current one mark to another mark by a command
signal having a sawtooth shape beginning with said reference
potential and decreases toward zero at said target position and to
supply a command signal when the transducer is in motion over a
second one of the marks toward a target one of the marks with
intermediate marks between the second and target ones of the marks
with a command signal having a predetermined maximum amplitude at
the second one of the marks and decreasing in amplitude to said
reference potential at a time when the transducer is at said target
position.
58. In a method of relatively moving first and second relatively
moveable members along a predetermined path of relative movement
from a first relative position to a second relative position
displaced from the first relative position a predetermined
distance, path location indicating marks on the second object;
including the steps of:
determining the number of said marks to be moved to achieve a
predetermined relative movement across said predetermined
distance;
if said number is greater than a first number, then firstly
relatively moving said members according to a predetermined
velocity profile until a relative position indicated by a second
plurality of marks from said second relative position is reached,
then changing control of the movement to a first predetermined
positioning control profile which ends in a stop-lock control at
said second relative position, said second plurality being less
than said first number but greater than two; and
if said number is less than said first number, then relatively
moving said members using a second predetermined positioning
control profile including a slope negative with respect to said
first predetermined positioning control profile and ending at said
stop-lock control.
59. In a positioning control for relatively positioning first and
second relatively moveable members, the first member carrying a
third member relatively moveable with respect to the first member,
said first member having a reference position at which the third
member is to be maintained, the positioning control operating to
relatively position the first and third members from a first
predetermined relative position to a target relative position, a
frame movably mounting said first and second members;
the improvement including, in combination:
a first position sensor on said first member and operatively
coupled to said third member for sensing the relative position
therebetween with respect to said reference position and generating
a relative position signal (RPS) for indicating displacement of
said third member from said reference position signal (RPS);
a second position sensor operatively coupled between said first and
second members for sensing the relative position therebetween and
for indicating the relative position as a position error signal
(PES);
fine and coarse actuators respectively mounted on said first member
and on said frame for respectively carrying said third and first
members along a common movement path on said second member;
position indicating marks on the second member and spaced apart
along said path in a sensible relationship to said second position
sensor such that the second position sensor supplies PES when the
third member is traversing said path which is a PES having a single
alternation intermediate two adjacent ones of said marks;
a first positioning circuit means coupled to said fine actuator for
actuating same to move said third member along said path from said
first predetermined relative position of said first and third
members toward the target relative position but not for
stop-locking the first and third members at said target relative
position;
a second positioning servo circuit means coupled to said fine
actuator for actuating same to move said third member along said
path over a limited number of said marks and for stop-locking the
third member over one of said marks, including marks respectively
at said first predetermined relative position and said target
relative position;
electronic switch means electrically interposed between both said
servo circuit means and said fine actuator for selectively coupling
one and only one of the servo circuit means to the fine
actuator;
a third positioning servo circuit means coupled to said coarse
actuator and to said first position sensor for moving the first
member along the path for returning the third and first members to
said reference relative position via either said first or second
position servo circuit means;
control means connected to said switch means for actuating same to
first connect said first positioning circuit means to said fine
actuator to move the third member from said first relative position
toward a second relative position between said second and third
members, said second relative position being displaced along said
path such that a first plurality of said marks are intermediate
said first and second relative positions, progress sensing means in
the control means and operatively associated with said path for
sensing progress of said movement of the third member along said
path and supplying a mode change signal when the movement of said
first member with respect to said second member is a second
plurality of marks greater than two and less than said first
plurality of marks from said second relative position, said control
means in response to said mode change signal actuating said switch
means to disconnect said first positioning circuit means from and
connect said second positioning servo circuit means to said fine
actuator.
Description
DOCUMENT INCORPORATED BY REFERENCE
Co-pending commonly assigned application for patent Fennema, Ser.
No. 123,675, filed Nov. 23, 1987 (TU987004) shows a position servo
circuit preferably used with the present application.
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to position control systems; more
particularly, position control systems having particular advantages
when used in record storage disk recording and player
apparatus.
2. Discussion of the Prior Art
Transducer positioning systems, particularly those used with
magnetic or optical disk recording and player apparatus, have used
a so-called velocity loop for long transducer motions, termed
seeks, i.e., seeks traversing a large number of concentric circular
record tracks. The velocity positioning servo mode is optimally
switched to a track-following positioning servo mode at one-quarter
track pitch from a target track. Such a track-following control may
be compared to a "stop-lock" positioning control in other
applications of positioning control systems. The track-following
position servo then positions the transducer to faithfully track or
follow the target track. In a subsequent seek operation, the
track-following position loop is interrupted to return to either a
velocity loop, a second positioning loop, or an open loop
"bang-bang" servo for moving the transducer to yet another target
track. It has been found that when the inter-track spacing is
reduced to obtain higher track densities, then overshoot and
track-settling problems become more acute when the servo mode is
switched from velocity mode to track-following mode at the
one-quarter track pitch from the target track. Accordingly, it is
desired to provide for a more reliable and faster transition from
velocity to positioning servo mode which results in faithful yet
rapid track-following mode for carried signal transducers.
Many optical recorders have a goal of high performance at low cost.
Accordingly, a so-called fine servo or fine actuator is carried
upon a head-carrying arm which is moved by a coarse actuator.
Typically, the fine actuator has high frequency response
characteristics and provides for rapid and short distance
positioning of the transducer with respect to a track being
followed or for moving from one track to a second target track,
which may be an adjacent track. The coarse servo which positions a
relatively large mass head-carrying head arm, as well as the fine
actuator, typically has frequency characteristics for handling the
longer moves for optimizing the relationship for top performance
between the fine and coarse actuators. The servo systems provide
for relative positioning of the fine actuator with respect to the
coarse actuator to a central or reference position. Such
arrangements have been colloquially called "piggy back" carriage
servo systems.
The application of such a "piggy back" carriage system is not
limited to disk recorders. Actually, the concept was established
many years ago for a pattern following or template-controlled
coarse-fine positioning servo mechanism. Such an arrangement
enabled higher production rates of a pattern controlled machine,
such as welding machines or cutting machines. The carried fine
actuator rapidly responds to sharp changes in the pattern such that
the welding or cutting operation faithfully follows the guiding
pattern template while only overcoming minimal inertia of the
pattern controlled machine mechanisms. Gardiner, U.S. Pat. No.
2,717,979 shows such an arrangement. Gardiner teaches that the fine
actuator, which Gardiner terms a topping servo, is controlled by
the absolute positioning of the pattern template; while the coarse
servo (called the main servo by Gardiner) is slaved to (always
follows) the positioning of the topping servo. This arrangement
means that the rapid responding topping servo controls the pattern
controlled machine while the main servo follows the motions of the
topping servo for maintaining the topping servo in an optimumal
position with respect to the main servo controlled carriage;
thereby maximizing the range of operation of the topping servo. The
Gardiner positioning servo arrangement is also shown in Meyer, U.S.
Pat. No. 4,627,039.
McIntosh et al., U.S. Pat. No. 3,924,268 and Merritt et al, U.S.
Pat. No. 4,513,332 show magnetic disk recorders having piggy-back
arrangements which are servo position controlled for optimizing the
relative position of the fine actuator with respect to the coarse
actuator. Simons, in U.S. Pat. No. 3,924,063 shows yet another
coarse-fine control wherein the fine actuator is permitted to move
over a predetermined minimum distance before a coarse actuator
operation is invoked. Van Winkle in U.S. Pat. No. 4,191,981 shows
fast and slow servo positioning mechanisms in a magnetic multiple
disk recorder in which the slow servo mechanism is slaved to the
fast servo mechanism; the latter arrangement is not a piggy-back
arrangement.
Coarse and fine actuator controls are also widely found,
particularly in magnetic disk recorders, for controlling a single
carriage. In many instances, the fine control is a position
responsive servo while the coarse or seek control is a velocity
responsive servo. In some instances, both servo controls use
position responsive controls. Examples of such coarse-fine controls
of both types are shown in Svendsen U.S. Pat. No. 4,268,785; Kaser
et al., U.S. Pat. No. 4,032,984; Case, U.S. Pat. No. 4,103,314;
Sordello, U.S. Pat. No. 3,458,785; and Johnson, U.S. Pat. No.
4,333,117. Coarse and fine controls are also shown for optical
recorders by van Rosmalen in U.S. Pat. No. 4,425,043 and by Janssen
et al. in U.S. Pat. No. 4,561,081.
Not all positioning systems employ two separate servo mechanism
control loops. An example of a single loop control for both track
seeking and track-following is shown by Matla et al. in U.S. Pat.
No. 4,217,612. Matla et al. employ a single position mode servo
which includes an analog summer circuit, which during the
track-following mode, has a control input of a reference potential.
For track switching or seeking, a track increment generator
supplies an input central signal to the summer circuit for
actuating the loop to move the head or transducer carriage radially
of a disk to another track position. In particular, see FIGS. 1 and
7 in this reference for a single loop controller which not only
track follows, but uses the same track-following loop for moving a
transducer in a seek operation over a plurality of concentric
record tracks.
Newell, in U.S. Pat. No. 2,800,769, shows a gun-directing servo as
yet another coarse-fine control. The fine control controls the
hydraulic speed gear of the gun directing or laying servo similar
to track-following in a disk recorder. A coarse control is
responsive to an input signal for supplying a relatively large
servo driving signal. This driving signal is coupled to a switch in
the servo loop which switches between fine and coarse controls.
When the coarse output signal is relatively large, the switch
responds to the large signal amplitude to switch from the fine to
the coarse control. When the output signal of the coarse control is
reduced below a given threshold, then the switch returns the servo
from the coarse control to the fine control. The gun-directing
servo apparently has a 360 degree rotational range. The fine
control positions the hydraulic speed gear within a 21/2 degree
error, which is about 0.7% of the positioning range. Based upon
this small position error range of the fine control, it is believed
that this fine control corresponds favorably to track-following in
disk recorders. Another aspect of the Newell arrangement is a rate
sensor which supplies a rate signal for assisting in deceleration
of the hydraulic speed gear toward the target rotational
position.
Optical recorders of both the record disk and record sheet (also
termed tablet or chip) type have employed servo positioning wherein
the light phase between adjacent tracks of optical indicia are
reversed; that is, between adjacent record tracks 1 and 2 a black
or opaque line extends for controlling the servo positioning.
Between adjacent tracks 2 and 3, a transparent line extends between
these adjacent tracks. Between tracks 3 and 4, the opaque line is
repeated, etc. Track following uses a grey scale representing a mid
point between the opaque and transparent track guiding lines. The
reversal relationship of the opaque and transparent lines
constitute a phase reversal in positioning control. Such an
arrangement is shown by King et al. in U.S. Pat. No. 2,843,841.
King et al. shows interrupting track-following mode for seeking to
an adjacent track, also termed "track jumping". According to King
et al., an open-loop pulse is applied to the positioning servo
circuits for moving an optical radiation beam toward an adjacent
target track. The open-loop pulse is designed to move the beam just
over one-half way between two adjacent tracks. At this time, the
phase of the servo circuits is reversed and the open loop pulse
terminates. At this time, the track-following servo takes over the
control of the radiation beam positioning for moving it to the
target adjacent track in a track-following mode. Jensen, in U.S.
Pat. No. 3,473,164, uses the arrangement for servo positioning
shown in King et al., but uses a different servo control mechanism
for moving the radiation beam from one track to an adjacent track.
Reversal of the servo phase is also employed by Jensen for jumping
the radiation beam to an adjacent track.
Even with all of the above variations of servo positioning of a
transducer or work object with respect to a record element (disk,
sheet or other work element), it is desired to provide for a more
rapid and faithful servo positioning, particularly for data
recorders having extremely high track densities. It is desired to
provide for a more faithful so-called track "capture" such that the
servo mode can switch from seeking to track-following more rapidly
and reliably. A problem found in using high track densities is that
as the transducer is radially moved slower and slower, the disk
run-out caused by eccentricity of the disk with respect to its
rotating spindle, causes radial motions of the track with respect
to the slow-moving transducer. Accordingly, an engineering problem
of faithfully measuring radial speed of the transducer with respect
to the tracks (which are relatively radially moving inward or
outward, depending upon eccentricity) becomes more difficult as
well as faithful counting of the track crossings for precisely
determining radial position with respect to the radially moving
tracks, becomes more difficult. It is also desired to use a
velocity servo loop for the long seeks.
SUMMARY OF THE INVENTION
A fine actuator/carriage is mounted for relative movement upon a
coarse actuator/carriage for positioning movement along a path over
a set of serially arranged positioning indicia. In a disk recorder,
the positioning indicia can be record tracks on a disk surface.
Such record tracks can be indicated by grooves in the record
surface, opaque/transparent areas and the like. In every seek for
traversing more than a certain number of indicia or tracks, a
velocity servo circuit controls the motion of the fine actuator for
rapidly positioning the fine actuator with respect to the
positioning indicia. A position responsive servo circuit is also
operatively connected to the fine actuator for maintaining position
of an object (transducer or work element) carried on a fine
actuator carriage with respect to a first position-indicating
indicium on track in the positioning path. The position servo
circuit includes short movement means for causing the fine actuator
to move the carried object over not more than said first number of
the positioning indicia to another indicium from the first
indicium. The short movement means in the position servo circuit
extends the range of operation of the position servo circuit from
operating with but one indicium to a range of several indicia.
Switching means are connected to the two servo circuits for
selectively connecting the servo circuits to the fine actuator. For
maintaining the fine actuator over the first indicium and for short
seeks or movements to other indicia within a predetermined range,
the position servo circuit is actuated. For longer seeks or
movements over a large number of position indicia, the velocity
circuit is actuated to move the work object or transducer over the
indicia. When the work object is moving toward a target indicium
over an indicium at about the outer extent of the range of
operation of the position servo circuit with respect to a target
indicium, the switching means switches the control of the
positioning from the velocity servo circuit to the position servo
circuit for completing the seek and to achieve later track capture
to a target indicium at one-quarter pitch from the target
indicium.
In a preferred mode, the position servo circuit is actuated for
following the velocity servo circuit positioning control such that
when the outer extremity of the position servo circuit range is
reached, the servo drive signal from both the position and velocity
servo circuits, are similar or identical. Switching from velocity
to position mode control then becomes easy for enabling the
position servo circuit to rapidly and faithfully finish the seek to
the target indicia. In a particular embodiment, a comparator is
connected to both position and velocity servo circuits for
comparing their respective output servo drive signals. When the
signals are identical, the comparator actuates the switching means
to switch from the velocity mode to the position servo mode.
In another aspect of the invention, the velocity and position servo
circuits operate to control a fine actuator carried by a coarse
actuator. The coarse actuator is slaved to the positioning of the
fine actuator through a position control servo circuit. The servo
drive signal to the fine actuator is fed forward through a coupling
circuit to the coarse positioning circuit for enabling more
faithful following of the coarse actuator to the fine actuator for
movements during seeking.
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a simplified block and diagrammatic showing of an optical
disk recorder in which the present invention may be advantageously
employed.
FIG. 2 is a simplified block and diagrammatic showing of a
preferred implementation of the invention in the FIG. 1-illustrated
recorder.
FIG. 3 is a graph illustrating a seek operation controlled by the
FIG. 2-illustrated servo circuits.
FIG. 4 is a graph showing operation of the FIG. 2-illustrated
circuit switching from velocity to position modes of servo
operation.
FIG. 5 is a block and diagrammatic detailed showing of FIG.
2-illustrated servomechanism.
FIG. 6 is a simplified block diagram of a coarse position circuit
used in the FIG. 5 illustration.
DETAILED DESCRIPTION
Referring now more particularly to the appended drawing, like
numerals indicate like parts of structural features shown in the
various figures. Referring particularly to FIG. 1, a magnetooptic
record disk 30 is mounted for rotation on spindle 31 by motor 32.
The optical elements 33 on head arm carriage generally denoted by
number 34 move radially of disk 30. A frame 35 of the recorder
suitably mounts carriage 34 for reciprocating radial motions. The
radial motions of carriage 34 enable access to any one of a
plurality of concentric tracks or circumvolutions of a spiral track
for recording and recovering data on and from the disk. Linear
actuator 36, suitably mounted on frame 35, radially moves carriage
34 for enabling the track accessing. The recorder is suitably
attached to one or more host processors 37, such host processors
may be control units, personal computers, large system computers,
communication systems, image processing processors and the like.
Attaching circuits 38 provide the usual logical and electrical
connections between the optical recorder and the attaching host
processors 37.
Microprocessor 40 controls the recorder including the attachment to
the host processor 37. Control data, status data, commands and the
like are exchanged between attaching circuits 38 and microprocessor
40 via bidirectional bus 43.
Included in microprocessor 40 is a program, or microcode, stored in
read only memory (ROM) 41 and a data and control signal storing
random access memory (RAM) 42. This microcode enables
microprocessor 40 to operate the recorder and for effecting other
machine operations usually employed in signal recorders.
The optics of the recorder include an objective or focusing lens 45
mounted for focussing and tracking motions on head arm 33 by fine
actuator 46. Actuator 46 includes mechanisms for moving lens 45
toward and away from disk 30 for focussing as well as radial
movements for track-following and seeking movements. Numeral 47
denotes the two-way light path between lens 45 and disk 30. The
coarse actuator continually follows the fine actuator for
monitoring, as much as possible, the fine actuator at the radial
center of the fine actuator's range of motion. In magnetooptic
recording, magnet 48 provides a magnetic steering or bias field for
controlling the remnant magnetization direction of a small spot on
disk 30 illuminated by laser light from lens 45. The laser light
spot heats the illuminated spot on the record disk to a temperature
above the Curie point of the magnetooptic layer (not shown, but can
be an alloy or rare earth and transitional metals as taught by
Chaudhari et al., U.S. Pat. No. 3,949,387). This heating enables
magnet 48 to direct the remnant magnetization to a desired
direction of magnetization as the spot cools below the Curie point
temperature. Magnet 48 is shown as oriented in the "write"
direction, i.e., binary ones recorded on disk 30 normally are
"north pole remnant magnetization". To erase disk 30, magnet 48
rotates for moving the south pole adjacent to disk 30. Control 49
for magnet 48 is mechanically coupled to rotatable magnet 48 as
indicated by dashed line 50 to control the write and erase
directions. Magnet 48 may be replaced by an electric coil wherein
the electrical current directions are reversed for write and erase
operations. Microprocessor 40 supplies control signals over line 51
to control 49 for effecting reversal between the recording and
erasing magnetic directions.
It is necessary to control the radial position of the beam
following path 47 such that a track or circumvolution is faithfully
followed and that a desired track or circumvolution is quickly and
precisely accessed. To this end, focus and tracking circuits 54
control both the coarse actuator 36 and fine actuator 46. The
positioning of carriage 34 by actuator 36 is precisely controlled
by control signals supplied by circuits 54 over line 55 to actuator
36. Additionally, the actuator control by circuits 54 is exercised
by control signals travelling over lines 57 and 58 respectively for
focus and fine tracking and track switching actions of fine
actuator 46.
The focus and tracking position sensing is achieved by analyzing
laser light from laser 67 reflected from disk 30 over path 47,
thence through lens 45, through one-half mirror 60 and to be
reflected by half-mirror 61 to a so-called "quad" detector 62. Quad
detector 62 has four photo elements which respectively supply
signals on four lines collectively denominated by numeral 63 to
focus and tracking circuits 54. Quad detector 62 also includes
optics such as semicylindrical lens for optically processing the
light beam for effecting focus error detection. Aligning one axis
of the detector 62 with a track center line, track- following
operations are enabled. Focussing operations are achieved by
comparing the light intensities detected by diagonal photo elements
in the quad detector 62. Focus and tracking circuits 54 analyze the
signals on lines 63 to control both focus and tracking.
Recording or writing data onto disk 30 is next described. It is
assumed that magnet 48 is rotated to the desired position for
recording data. Microprocessor 40 supplies a control signal over
line 65 to laser control 66 for indicating that a recording
operation is to ensue. This means that laser 67 is energized by
control 66 to emit a high intensity laser light beam capable of
heating a spot on the medium to the Curie point for recording
signals; in contrast, for reading recorded signals, the laser 67
emitted laser light beam is a much reduced intensity which does not
heat the laser illuminated spot on disk 30 above the Curie point.
Control 66 supplies its control signal over line 68 to laser 67 and
receives a feedback signal over line 69 indicating the laser 67
emitted light intensity. Control 68 adjusts the light intensity to
the desired value, depending on whether a write operation is
occurring. Laser 67, a semiconductor laser such as a gallium
arsenide diode laser, can be modulated by data signals so the
emitted light beam represents the data to be recorded by such
intensity modulation. In this regard, data circuits 75 (later
described) supply data indicating signals over line 78 to laser 67
for effecting such modulation. This modulated light beam passes
through polarizer 70 (linearly polarizing the beam), thence through
collimating lens 71 toward half-mirror 60 for being reflected
toward disk 30 through lens 45. Data circuits 75 are prepared for
recording the microprocessor 40 supplied suitable control signals
over line 76. Microprocessor 40 in preparing circuits 75 is
responding to commands for recording received from a host processor
37 via attaching circuits 38. Once data circuits 75 are prepared,
data is transferred directly between host processor 37 over bus 77
into data circuits 75 through attaching circuits 38. Data circuits
75 also include ancillary circuits (not shown) relating to disk 30
format signals, error detection and correction and the like.
Circuits 75, during a read or recovery detection, strip the
ancillary signals from the readback signals before supplying
corrected data signals over bus 77 to host processor 37 via
attaching circuits 38. A data buffer (not shown) in data circuits
75 provides data buffering, as is well known for DASDs. During a
reading operation, MO detector 79 converts the two phases of
polarized light into data signals; the data signals are fed to data
circuits 75.
Reading or recovering data from disk 30 for transmission to a host
processor requires optical and electrical processing of the laser
light beam from the disk 30. That portion of the reflected light
(which has its linear polarization from polarizer 70 rotated by
disk 30 recording using the Kerr effect) travels along the two-way
light path 47, through lens 45 and half mirrors 60 and 61 to the
data detection portion 79 of the head arm 33 carried optics.
Half-mirror or beam splitter 80 divides the reflected beam into two
equal intensity beams both having the same reflected rotated linear
polarization. The half-mirror 80 reflected light travels through a
first polarizer 81 which is set to pass only that reflected light
which was rotated when the remnant magnetization on disk 30 spot
being accessed has a "north" or binary one indication. This passed
light impinges on photocell (P) 82 for supplying a suitable
indicating signal to differential amplifier 85.
When the reflected light was rotated by a "south" or erased pole
direction remnant magnetization, then polarizer 81 passes no or
very little light resulting in no active signal being supplied by
photocell (P) 82. The opposite operation occurs by polarizer 83
which passes only "south" rotated laser light beam to photocell (P)
84. Photocell 84 supplies its output signal indicating its received
laser light to the second input of differential amplifier 85. The
amplifier 85 supplies the resulting difference signal (data
representing) to data circuits 75 for detection. The detected
signals include not only data that is recorded, but also all of the
so-called ancillary signals as well. The term data as used herein
is intended to include any and all information-bearing signals,
preferably of the digital or discrete value type. Other forms of
information signal recording may be used.
The rotational position and rotational speed of spindle 31 is
sensed by a suitable tachometer or emitter sensor 90. Sensor 90,
preferably of the optical-sensing type that senses dark and light
spots on a tachometer wheel (not shown) of spindle 31, supplies the
"tach" signals (digital signals) to APS (angular or rotational
position sensing) circuit 91 which detects the rotational position
of spindle 31 and supplies rotational information-bearing signals
to microprocessor 40. Microprocessor 40 employs such rotational
signals for controlling access to data storing segments on disk 30
as is widely practiced in the magnetic data storing disks.
Additionally, the sensor 90 signals also travel to spindle speed
control circuits 93 for controlling motor 32 to rotate spindle 31
at a constant rotational speed. Control 93 may include a crystal
controlled oscillator for controlling motor 32 speed, as is well
known. Microprocessor 40 supplies control signals over line 94 to
control 93 in the usual manner.
For the FIG. 1-illustrated embodiment to follow the "Gardiner"
servo positioning approach, sensor 97 is mounted on carriage 34 and
is operatively coupled to fine actuator 46, as diagrammatically
indicated by dashed line 98. The operative coupling is preferably
an optical communication path. For example, fine actuator 46
carries an opaque/transparent flag or reflective/transparent flag.
Sensor 97 shines a light toward the flag with the transmitted or
reflected light being intercepted by photocells (not shown) of
sensor 97, which faithfully indicate the relative radial position
of fine actuator 46, with respect to carriage 34. Sensor 97
includes circuitry of known design that translate the intercepted
light into a relative position indicating signal (RPS) traveling
over electrical line 99 to focus on tracking circuits 54. The
relative position signal on line 99 actuates focusing and tracking
circuits 54 to slave the movement of head carriage 34 by actuator
36 through signals supplied over line 55, as will be later
detailed.
Referring more particularly now to FIG. 2, focus and tracking
circuits 54 operation is described with respect to the control of
actuator 46 and the position controls of working object or lens 45
with respect to the record disk or work element 30. Focus and
tracking circuits 54 include two track servo circuits, 105 and 106,
respectively of the position and velocity servo type. Position
servo circuit 105 not only controls actuator 46 to ensure that lens
45 faithfully follows (also termed a "stop-lock" mode) a record
track on disk 30, but also has an extended range of radial
operation for controlling lens 45 positioning over a plurality of
record tracks of disk 30. For example, the extended range of
position servo circuit 105 may be plus and minus ten tracks of a
current track being followed. With respect to a current track being
followed, for seeks of radiation beam 47 over a number of record
tracks greater than ten, velocity servo circuit 106 controls
actuator 46. Electronic switch 107 selectively transfers the servo
drive output signals of circuits 105 and 106 over line 57 to fine
actuator 46. Switch 107 is controlled such that on a long seek
(more than ten tracks are traversed) the switch electrically
connects input terminal V, which carries the velocity servo drive
signal from line 109, to line 57. When radiation beam 47 is
approximately ten tracks from the target track, then switch 107
disconnects terminal V from line 57 and reconnects terminal P to
transfer the position servo circuit 105 servo drive signal over
line 57 to actuator 46. This latter connection enables the position
servo circuit 105 to utilize its extended range of operation to
faithfully and quickly complete the long seek to the target track.
Switching from terminal V to terminal P is performed when the drive
signals from circuits 105 and 106 are approximately equal. To
effect this switching synchronization, power compare circuit 108
receives the respective servo drive signals over lines 109 and 110
for comparing the RMS (root mean square) parameter of the signals.
Such comparison is a well known technique and not described for
that reason. When power compare circuit 108 detects approximate
equality of the two servo drive signals, it emits a mode-changing
signal over line 113 for actuating switch 107 to disconnect
terminal V and reconnect terminal P to output line 57. Line 113
also extends to velocity servo circuit 106 for quiescing the signal
compensation circuits therein, as is well known.
Compare circuit 108 is activated when laser beam 47 is approaching
the outer extremity of the range of operation of position servo
circuit 105. Such positioning is indicated by track counter 111
which was initially set by microprocessor 40 to indicate a
direction of motion and number of record tracks (position indicia)
to be traversed during the seek. Cable (electronic bus having a
plurality of electrical circuit lines) 116 carries the distance to
go and direction signals to track counter 111, as well as to
velocity servo circuit 106. Velocity servo circuit 106 responds to
the distance to go signal to set up a mode of operation optimizing
the seek operation. Track counter 111 detects when the track count
is approaching ten for supplying a compare circuit 108 activating
signal over electrical line 112. Compare circuit 108 responds to
the line 112 signal for comparing the servo drive signals on lines
109 and 110, as above described. Line 112 also extends to position
servo circuit 105 for activating same to follow the positioning of
radiation beam 47 as effected by velocity servo circuit 106; that
is, circuits 105 now produce a servo drive signal for use by a
compare circuit 108. Track counter 111 supplies the current track
count over cable 114 to microprocessor 40 and position servo
circuits 105 and 106. Circuits 105 and 106 use the track count in a
usual manner.
A track seek operation is initiated under control of microprocessor
40. Microprocessor 40 supplies a seek initiate signal over line 115
which extends to switch 107 and circuits 105 and 106. For a short
seek, the seek signal keeps switch 107 connected to the P terminal
such that position servo circuit 105 controls the seek operation
through its extended range of operation. The line 115 seek signal
activates position servo circuit 105 in the short seek, while in a
long seek quiesces a later shown signal compensation circuit in
preparation for the track seek mode switch. The seek command also
goes to velocity servo circuit 106 for activating same for a long
seek, as will become apparent. All seeks are completed by position
servo circuit 105 entering a track-following or stop-lock mode.
When the track-following operation begins, circuit 105 supplies a
seek complete signal over line 117 to microprocessor 40.
Coordination of the operation of the track counter 111 and circuits
105 and 106 is through the position error signal supplied over line
63 from detector and optics 79. FIG. 4 illustrates the spatial
synchronization of circuit operation using the PES signal not only
for track-following and position servo seeking, but also track
counting.
FIG. 3 broadly illustrates a seek operation using both velocity and
position servo circuits in a move from a current track centerline C
to a target track centerline T. The graph is in the space domain;
does not illustrate the signals in any time base. The beam 47
velocity or speed curve with respect to the recording surface of
disk 30 is illustrated by line 120. Velocity, of course, begins at
zero and proceeds to a maximum value whereas deceleration begins
toward and follows line 122 to track T. The velocity servo circuit
106 servo drive signal is represented by dashed line 121 which
effects the resultant velocity signal curve 120. During
deceleration, numeral 122 designates that the velocity curve 120
and velocity servo circuit 106 servo drive signal are coincident.
When track counter 111 contains a count TK equal to K (K is ten in
the preferred embodiment), track counter 111 emits its compare
enable signal over line 112 which corresponds to dashed line 123 in
FIG. 3. At this time, the position servo circuit 105 servo drive
signal POS is turned on as indicated by signal wave 124. Signal
wave 124 follows the deceleration curve 122 at spatial line 125.
Compare circuit 108, detecting that the RMS value of POS signal 124
is equal to the velocity servo circuit 106 servo drive signal VEL
on line 109, actuates switch 107 to disconnect the V terminal from
and connect the P terminal to actuator 46. Since the RMS amplitudes
of the signals on line 109 and 110 are substantially identical, no
transients are introduced into the operation of actuator 46. The
power compare circuit 108 signal on line 113 also quiesces the
operation of velocity servo circuits 106 as indicated in FIG. 3 by
dashed line 126.
The detection of the position of beam 47 during the track seek is
enabled by the line 63 (position--tracking--error signal) PES 129,
as best seen in FIG. 4. PES 129 is shown in the spatial domain as a
sine wave with positive going zero crossings at the various track
center lines T-10 through T+1. The negative going zero crossing
occurs at the midpoint between two adjacent track centerlines.
Track counter 111 includes a zero crossing detector for counting
the track crossings. Two zero crossings are counted for each
traversal of a track. As later described, PES 129 is differentiated
to create signal PES' 130. Of course, differentiation causes an
effective phase shift of 90 degrees. For position servo circuit 105
to fully utilize the position error signal, the negative signal
portions 131, also shaded, are inverted to become positive between
the successive zero crossings 156 intermediate adjacent track
centerlines. Zero crossings 156 correspond to the one-quarter point
between adjacent tracks. POS signal 124 is enlarged in FIG. 4 for
showing its relationship to PES 129 and PES' 130. Dashed line 123
of FIG. 3 is shown as being coincident with the centerline of track
T-10, i.e., ten tracks to go to target track T, no limitation
thereto intended. Dashed line 125 is shown as occurring
asynchronously with respect to the PES 129, i.e., independent of
the actual radial position of beam 47 with respect to any track
centerline. If desired, the transition can be synchronized to such
track centerlines. The line 112 signal is shown immediately below
the signal POS 124. At the one-quarter track pitch position before
beam 47 reaches the track centerline of track T, the line 112
signal is removed.
Position servo circuit 105 not only provides for terminating a long
seek and track-following, but also provides for jumping one track,
i.e., to an adjacent track as well as providing short seeks up to
ten tracks from a current track. Numeral 133 designates such a
single track jump from target track T to new target track T+1. The
mode switching from track-following to jumping occurs at maximum
amplitude of PES 124. Signal 133 ramps as a sawtooth wave with
maximum amplitude at about the centerline of T+1 whereupon it
relaxes to zero amplitude. The operation is such that the POS
control signal 133 and a later described error signal which results
in the servo drive closely follows signal 133 such that relatively
small signal amplitudes are always present within position control
circuit 105. See the Fennema application for patent, supra.
Referring now more particularly to FIG. 5, the detailed discussion
is provided for a currently preferred embodiment of the invention.
The microprocessor 40 seek command line 115 actually consists of
two electrical signal lines, 138 and 139. Electrical line 138
carries a seek command irrespective of the length of the seek for
setting seek latch or flip-flop 140. Seek latch 140 being set to
the seek condition, conditions position servo circuit 105 for its
extended range operation, as will become apparent. The line 139
signal identifies a seek longer than ten tracks, i.e. a long seek.
The long seek signal travels over line 139 for setting velocity
mode latch 141, which in turn actuates electronic switch 107 to
move from the track-following P position to the long seek velocity
drive position V. Line 139 also extends to velocity servo circuit
106 for enabling the circuit for performing a long seek operation.
Circuit 106 can be of any velocity circuit design and is not
described for that reason. The long seek signal activates circuit
106 to receive and respond to the commanded distance-to-go number
of tracks supplied by microprocessor 40 over cable 116 as well as
the current number of tracks to go as supplied by track counter 11
over cable 114. Velocity mode latch 141 supplies a first output
indicating the velocity mode over line 142 as one condition of
actuation of power compare circuit 108. To this end, analog AND
circuits 143 and 144 are electrically interposed respectively
between lines 110 and 109. The enablement of AND circuits 143 and
144 is completed by the line 112 signal (FIG. 4) for supplying an
input to compare circuit 108, which operates as previously
described.
Track counter 111 responds to the line 63 PES signal 129 through
zero crossing detector (OX) circuit 147. OX circuit 147 supplies
two pulses for each track crossing to track counter 111 which
responds thereto for down counting the tracks to go. A direction
signal derived from the detector (not shown) may be used for
controlling the directional count of track counter 111. In the
present embodiment, because of the extended range of operation of
position servo circuit 105, the velocity of beam 47 with respect to
the surface of disk 30 always has a sufficient velocity to
accommodate run-out. The zero crossing signals from OX circuit 147
also are directed to AND circuit 148 for resetting velocity mode
latch 141 to the position mode state. AND circuit 148 is enabled to
pass a single zero crossing pulse by the line 112 signal and
velocity mode latch 141 being set to the velocity mode state (VS).
The AND circuit 148 output signal travels over line 149 thence
through AND circuit 150, as enabled by the line 113 signal received
from compare circuit 108, to reset velocity mode latch 141. This
action by the connection over line 145 to electronic switch 107
actuates the switch to disconnect terminal V and line 109 from line
57 and reconnect terminal P and line 110 to line 57. Line 145 also
extends to later described EPROM 170 within circuit 105.
The detailed operation and construction of position servo circuit
105 are next described. The line 63 PES signal 129 is
differentiated by differentiator (DIFF) 153 to produce the PES'
130. PES' 130 goes to polarity reversing circuit 154. Circuit 154
includes zero crossing detector (OX) 155 which detects the zero
crossings 156 of PES' 130. AND circuit 157 receives the zero
crossing 156 indicating signals and when enabled by the seek signal
on line 160 and the line 112 signal from track counter 111,
actuates electronic switch 158 to disconnect the plus terminal from
its output to signal integrator 164 and connect the minus terminal
thereto. The plus terminal directly connects the output of
differentiator circuit 153 to integrator 164. The minus terminal
connects the output of differentiator 153 through signal inverter
or polarity reversing amplifier 159 to integrator 164. Inverter 159
inverts the polarity of negative signal portions 131 of PES' 130.
The unipolar PES' signal is integrated by integrator 164 then
applied to analog summer circuit 165, thence compensator 166.
Compensator 166 supplies the position servo circuit servo drive
signal over line 110. Compensator 166 is designed, using known
techniques, to enable maximum bandwidth with adequate stability
margins, as is common in the servo art.
The control signal for summer 165 to be mixed with the position
(tracking) error signal from integrator 164 is generated by a
self-sequencing EPROM 170. EPROM 170 is activated by the line 145
signal indicating the position servo mode. Track counter 111
supplies the number of tracks to go to EPROM 170 over cable 114.
When track counter 111 has a zero track count, then EPROM 170
causes position servo circuits 105 to be in a track-following mode.
The line 160 seek signal also goes to EPROM 170 for ensuring that
circuit 105 remains in the track- following mode in the absence of
a seek signal irrespective of the numerical contents of track
counter 111. EPROM 170 is programmed to generate a position drive
signal in the digital form which is supplied over cable 172 to
digital analog convertor (DAC) 171. DAC 171 in turn supplies the
analog position command signal to summer 165. EPROM 170 includes an
electronic table representing the desired position reference
signals in digital form for each track count received from track
counter 111. The generation of such tables in operations of EPROM
are sufficiently well known so that detailed description may be
dispensed with.
The seek complete signal to be supplied over line 117 to
microprocessor 40 is generated by AND circuit 175. The one-quarter
track position from target track T is indicated by the zero
crossing signal carried over line 176 from OX 155. EPROM 170
supplies a signal over line 177 indicating that the position of
beam 47 with respect tot rack T centerline is approaching the
one-quarter track pitch position, as shown in FIG. 4. AND circuit
175 responds to the two signals to supply the seek complete signal
over line 117 which not only goes to microprocessor 40 but also
resets seek latch 140 to the off or non-seek condition. Line 177
signal from EPROM 170 corresponds to track counter 111 indicating
to EPROM 170 that the number of tracks to go is unity, i.e., the
next track is target track T.
Sensor 97 supplies its error signal EPR over line 99 to coarse
position servo 180. Line 181 connects the servo drive signal at
actuator 46 to coarse position servo 180 as a feed forward signal
to ensure that coarse actuator 36 moves head carriage 34 in an
optimum manner such that fine actuator 46 follows the center line
of a track as closely as possible.
FIG. 6 is a simplified diagram of a coarse position servo circuit
180. Analog summer 185 receives the sensor 97 position error signal
over line 99. The position error signal is compared with ground
reference potential at ground point 186. The summer 185 error
signal is supplied through compensator 187 to analog summer 188.
Summer 188 combines the feed forward signal on line 181 with the
compensated error signal and supplies a coarse position drive
signal through amplifier 189 to coarse actuator 36. Circuit 182 is
electrically interposed between line 181 and summer 188 for
providing smoothing and amplitude translation of the line 181
carried signal from fine actuator 46.
While the invention has been particularly shown and described with
reference to its preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention.
* * * * *